Unennoctium
Unennoctium, Ueo, is the temporary name for element 198. NUCLEAR What follows is based on a first-order, liquid-drop assessment of where the outer boundary of the nuclear world is. Assume cautious values for how many neutrons a nucleus with 198 protons can bind (high neutron dripline) and how few it can have before it fissions immediately regardless of how much the structure it can develop stabilizes it (low must-fission curve). Assume, too, that anything that lasts long enough so that protons and neutrons can be treated as particles rather than collections of quarks (is causal) might be a nucleus. Under these conditions, Ueo isotopes are theoretically possible between Ueo 540 and Ueo 874 (see "The Final Element", this wiki). Ueo 540 through Ueo 695 are expected to decay by beta emission if they don’t fission quickly. Above that value of A, the confident neutron dripline, drops may decay by neutron emission before they can fission. (Structural correction does not affect neutron emission.) Isotopes lighter than Ueo 568 need more than twice the structural correction energy needed to prevent fission in worst-case nuclei in the A = 480 region(1). Predicting whether or not the structure a nuclear drop can develop will allow it to survive for the 10^-14 sec required for it to bind an electron and so become an atomic nucleus is not usually possible at this time. Neutron shell closures have been predicted at N = 524, and 406(2),(3),(4). The isotope Ueo 722 requires 1.5 MeV of structural correction, but lies 4% above the confident dripline, which means isotopes between Ueo 712 and Ueo 727 are energetically favored but may decay by neutron emission. The isotope Ueo 604 requires 14.5 MeV of structural correction, which may stabilize some isotopes in the Ueo 594 to Ueo 609 band, if the N = 406 closure is strong. Long beta-decay half-lives are possible in this band, which may allow decay by alpha emission. Isotopes between Ueo 568 and Ueo 792 have some probability of existing. Outside this band, isotopes of Ueo are nearly impossible. ATOMIC Electron structure of Ueo has not been studied closely, but it is likely to differ significantly from the conventional orbitals found in lower-Z nuclei. While only the innermost electrons would be qualitatively different, other electrons are likely to be quantitatively different from those in lower-Z atoms. Ueo is also large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Ueo to have different electronic structures. (That means it is no longer an element in the chemical sense.) Predictions of atomic or chemical properties of Ueo are risky. FORMATION Ions of this element may form when material from roughly 1 km depth is ejected from a disintegrating neutron star during a merger. It is probably impossible for lighter isotopes to form in this way. Fusion or multinucleon transfer reactions in the polar jets emanating from a neutron star or black hole might produce lighter isotopes, including those in the Ueo 712 to Ueo 727 and Ueo 594 to Ueo 609 bands. Quantities amount to a few atoms per star at best. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v. 68, no. 7, pp 1133-1137; 2005. 3. “Search for Superheavy Elements Among Fossil Fission Tracks in Zircon”; J. Maly & D.R. Walz; Stanford Linear Accelerator Center publication SLAC-PUB-2554; July 1980. 4. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. (12-06-19)